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Second Derivatives and Laplace Operator

### 2.13: Second Derivatives and Laplace Operator

The first order operators using the del operator include the gradient, divergence and curl. Certain combinations of first order operators on a scalar or vector function yield second order expressions. Second-order expressions play a very important role in mathematics and physics. Some second order expressions include the divergence and curl of a gradient function, the divergence and curl of a curl function, and the gradient of a divergence function.

Consider a scalar function. The curl of its gradient can be written as follows: For a vector function, the divergence of a curl can be expressed as follows: The curl of a gradient function and the divergence of a curl function are always zero.

The divergence of the gradient of a scalar function can be expressed as follows: The Laplacian is analogous to the second-order differentiation of the scalar quantities. It describes physical phenomena like electric potentials and diffusion equations for heat flow.

The divergence of gradient and the curl of a curl are mathematical constructs. Lagrange's vector cross-product identity formula relates both to a vector Laplacian. The vector Laplacian is obtained by directly applying the scalar Laplacian to each of the scalar components of a vector.  X 